1. Field of the Invention
The present invention relates generally to a method and apparatus for correctly estimating or equalizing data sequences and more specifically to a method and apparatus which is capable of following rapid changes in channel characteristics with a very small number of data processing steps. The present invention is well suited for use in mobile communications systems for example.
2. Description of the Prior Art
It is known in the art of radio communications that a receiver using maximum-likelihood sequence estimation exhibits an extremely low error rate, particularly in the case of intersymbol interference. This maximum-likelihood sequence estimation technique is disclosed in an article published in IEEE Transaction on Information Theory Vol. IT-18, No 3, May 1972 entitled "Maximum-Likelihood Sequence Estimation of Digital Sequences in the Presence of Intersymbol Interference" by G. D. Forney Jr (prior art paper 1).
In order to meet the situation wherein channel impulse responses vary with respect to time, an adaptive maximum-likelihood receiver has been proposed and disclosed in an article published in IEEE Transactions on Communications Vol. Com-22, No. 5. May 5, 1974 entitled "Adaptive Maximum-Likelihood Receive for Carrier-Modulated Data-Transmission Systems" by Gottfried Ungerboeck (prior art paper 2). This type of apparatus and method utilizes a known training sequence, which is previously sent to a receiver in order to enable initial estimation of channel response of later data transmission. Following this, a decision sequence is generated from a sequence estimator with a predetermined delay using an adaptive algorithm. However, this arrangement algorithm is unable to follow very rapid changes in channel characteristics.